Abstract
AbstractBedrock rivers represent a hydrogeological environment in which surface water flows along an exposed bedrock surface. Studies of hyporheic exchange have exclusively involved rivers composed of unconsolidated fluvial sediments, leaving a critical knowledge gap. This study evaluates the conditions that could support bedform‐scale hyporheic exchange within a fractured sedimentary bedrock river based on field data collected near Guelph, Canada. Hyporheic exchange at the bedform‐scale was evaluated by numerically modeling the migration of a conservative solute tracer through a bedrock riverbed within a two‐dimensional vertical cross‐section along the flow direction. A stochastic discrete fracture‐matrix framework was developed to represent measured subsurface fractured bedrock properties, producing a probabilistic distribution of potential hyporheic exchange pathways. Flow and transport model results indicate that: (a) bedform‐scale hyporheic exchange within a bedrock river exists with high fluid flow velocities in fractures, yet long solute residence times due to diffusion across the fracture‐matrix boundary; (b) the coincidence of fractures and hydrodynamic head gradients across the riverbed controls the spatial extent of bedform‐scale hyporheic exchange; and (c) the potential variability in hyporheic exchange residence time is large (i.e., tens of years) due to the inherent variability in facture network properties. Our field‐based numerical study indicates that the average (median) residence time may not be a good proxy for the potential natural attenuation capacity of a fractured sedimentary bedrock riverbed and that hyporheic exchange has the potential to emplace surface water contaminants within the fractured porous rock matrix that could become a long‐term source of trace contaminants.
Highlights
Bedrock rivers occur where surface water flows along an exposed bedrock surface. Tinkler and Wohl (1998) define bedrock rivers as reaches where a greater proportion (>50%) of the riverbed is an exposed rock surface or is covered by a thin alluvial veneer of weathered bedrock fragments that can be mobilized and removed during high flows
A field investigation between a bedrock river, Twenty Mile Creek, and a local aquifer in Smithville, Ontario, Canada by Oxtobee and Novakowski (2002) provided some early insights into groundwater-surface water (GW-SW) interactions in bedrock rivers. Their investigation revealed that groundwater discharge was extremely limited within Twenty Mile Creek, with more than 95% of the groundwater underflowing the creek during baseflow conditions. Their results indicated that groundwater discharge into Twenty Mile Creek was primarily through discrete fractures and that poor vertical fracture connectivity was responsible for the limited GW-SW exchange
This study evaluated the occurrence and spatial extent of hyporheic exchange within a sedimentary bedrock riverbed by quantifying the advection-diffusion interaction using a 2D discrete-fracture matrix numerical model based on parameters and conditions from a field research site
Summary
Bedrock rivers occur where surface water flows along an exposed bedrock surface. Tinkler and Wohl (1998) define bedrock rivers as reaches where a greater proportion (>50%) of the riverbed is an exposed rock surface or is covered by a thin alluvial veneer of weathered bedrock fragments that can be mobilized and removed during high flows. A field investigation between a bedrock river, Twenty Mile Creek, and a local aquifer in Smithville, Ontario, Canada by Oxtobee and Novakowski (2002) provided some early insights into GW-SW interactions in bedrock rivers Their investigation revealed that groundwater discharge was extremely limited within Twenty Mile Creek, with more than 95% of the groundwater underflowing the creek during baseflow conditions. A subsequent numerical model by Oxtobee and Novakowski (2003) indicated that groundwater discharge (or recharge) to the bedrock river depended on the aperture size of the discharging features and the vertical hydraulic head distribution within the fracture network Their studies were limited to groundwater discharge to surface water, Oxtobee and Novakowski (2002, 2003) emphasized the importance of characterizing fracture network properties when simulating interactions in fractured riverbed environments
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